Dendritic cells (DCs) are specialized cells of the immune system that serve as an essential bridge between the innate and adaptive immune system. DCs and other innate immune cells sense pathogens by their expression of different types of pathogen recognition receptors. Activation of these receptors triggers the innate immune response, which is the body's first line of defense, providing a rapid, but limited response to infections. However, DCs are unique from other immune cells in their ability to take up antigen and migrate to local lymph nodes where they present processed antigen to an organized pool of T lymphocytes. This initiates the adaptive arm of the immune response, which is the basis for immunological memory and vaccination. DCs are also implicated in other critical processes, including the induction of tolerance and tumor immunosurveillance. However, many questions about how DCs perform these functions and how they may be therapeutically modulated remain unanswered. The use of zebrafish as a model system represents a potentially powerful alternative to conventional approaches of studying the immune system. Previous studies have demonstrated that many of the essential components of the adaptive immune system, including B and T lymphocytes, are conserved in zebrafish. Furthermore, the Traver laboratory has recently identified a population of cells that share hallmark morphological and functional features of mammalian DCs. We propose to test the hypothesis the initiation of the adaptive immune response is mediated by DCs in zebrafish and thus conserved between fish and mammals. First, we will improve current methods identifying zebrafish DCs through genetic comparison with mammalian DCs. Second, since zebrafish lack obvious lymph nodes, we will try to identify lymph node-like organization in tissues by tracking the migration of DCs and interaction with T cells by confocal imaging. Finally, we will test the function of zebrafish DCs by deleting them in live animals and measuring the response to natural pathogens. If the proposed aims are achieved, we and others will be able to take full advantage of exciting features of the zebrafish model, including unique genetic tools, real-time live imaging, forward genetic screens, and drug screens, which will ultimately allow us to answer questions about how DCs fundamentally work and identify new therapies to target these functions.